Add a SpringBootVersion class like SpringVersion

Provide a static utility method to get the Spring Boot version based on
the implementation version of SpringApplication's package. Similar to
Spring Frameworks' SpringVersion class.

Fixes gh-2300

spring-boot/src/main/java/org/springframework/boot/ResourceBanner.java

 /*
- * Copyright 2012-2014 the original author or authors.
+ * Copyright 2012-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -106,7 +106,7 @@ public class ResourceBanner implements Banner {
 	}
 
 	protected String getBootVersion() {
-		return Banner.class.getPackage().getImplementationVersion();
+		return SpringBootVersion.getVersion();
 	}
 
 	private String getVersionString(String version, boolean format) {

spring-boot/src/main/java/org/springframework/boot/SpringBootBanner.java

 /*
- * Copyright 2012-2013 the original author or authors.
+ * Copyright 2012-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -50,7 +50,7 @@ class SpringBootBanner implements Banner {
 		for (String line : BANNER) {
 			printStream.println(line);
 		}
-		String version = Banner.class.getPackage().getImplementationVersion();
+		String version = SpringBootVersion.getVersion();
 		version = (version == null ? "" : " (v" + version + ")");
 		String padding = "";
 		while (padding.length() < STRAP_LINE_SIZE

spring-boot/src/main/java/org/springframework/boot/SpringBootVersion.java

+/*
+ * Copyright 2012-2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot;
+
+/**
+ * Class that exposes the Spring Boot version. Fetches the "Implementation-Version"
+ * manifest attribute from the jar file.
+ * <p>
+ * Note that some ClassLoaders do not expose the package metadata, hence this class might
+ * not be able to determine the Spring Boot version in all environments. Consider using a
+ * reflection-based check instead: For example, checking for the presence of a specific
+ * Spring Boot method that you intend to call.
+ *
+ * @author Drummond Dawson
+ * @since 1.3.0
+ */
+public class SpringBootVersion {
+
+	/**
+	 * Return the full version string of the present Spring Boot codebase, or {@code null}
+	 * if it cannot be determined.
+	 * @return the version of Spring Boot or {@code null}
+	 * @see Package#getImplementationVersion()
+	 */
+	public static String getVersion() {
+		Package pkg = SpringApplication.class.getPackage();
+		return (pkg != null ? pkg.getImplementationVersion() : null);
+	}
+
+}